Move digest email docs to a separate file and improve styling

docs/administration/CLI_tasks/digest.md

@@ -0,0 +1,15 @@
+# Managing digest emails
+Every command should be ran with a prefix, in case of OTP releases it is `./bin/pleroma_ctl digest` and in case of source installs it's `mix pleroma.digest`.
+
+## `test`
+
+```sh
+$PREFIX test <nickname> <since_date>
+```
+
+Send digest email since given date (user registration date by default) ignoring user activity status.
+
+Example: 
+```sh
+$PREFIX test donaldtheduck 2019-05-20
+```